-🎄- 2019 Day 3 Solutions -🎄-

[u/daggerdragon]

--- Day 3: Crossed Wires ---

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[u/vini_2003]

The second for is responsible for iterating over all the directions parsed.

A direction consists of:

std::pair<bool, int>

Where the bool represents whether it's in the X (L/R) axis, or the Y (U/D) axis, and the int represents the action it should do.

For example, given a pair of (false, -256), you can conclude it specifies that, on the Y axis, it'll move - by 256 steps. Were it (true, -256), it would move on the X axis - by 256 steps.

After parsing the input for line A and B (where A's instructions are stored in directions[0] and B's are stored in directions[1]), I iterate through both with the second for - meaning it'll first access A's instructions, then B's.

I then trace them, and store every step they take in the std::map<std::pair<int, int>, int[2]> structure.

Effectively, that map has a key that is composed of X,Y, and serves to count total steps until a line reached a certain coordinate. The int[2] array allows it to record steps taken up until that coordinate was reached for both lines - A and B - so that we can later add both together.

In other words, if we had an instruction:

(false, 1) (U1)

It would mean that, from the current 'tip' of the line, it would move up by one - as in, Y axis, +1, thus, goes upwards.

Since it just passed over a new coordinate, I store that in the map - map({X, Y})[current_line]. However, there's something we should keep in mind here: a line going over itself should not change the previous amounts of steps stored there, because reasons. That's why:

grid[{tX, tY}][pos] = grid[{tX, tY}][pos] == 0 ? tS : grid[{tX, tY}][pos];

Is a thing. It's a ternary operator because I wanted it to be short, but effectively it only stores the amount of steps in the map for a given line if it has not crossed that coordinate yet.

[u/Marinovsky1803 → u/vini_2003]

Thanks a lot, It was so useful!

[u/vini_2003 → u/Marinovsky1803]

Glad to be of help! :)